In recent years, according to high interest in environmental pollution and consolidated regulations, a water treatment technique that employs a filter membrane superior in view of completeness of separation of contaminants and compactness or the like has attracted attention.
As a process for production of a porous membrane used as a filter membrane, a non-solvent phase separation technique (refer to NPLs 1 and 2, for example) that uses a non-solvent phase separation phenomenon in which a polymer solution is phase-separated by a non-solvent to become porous is known.
As the non-solvent phase separation technique, a wet spinning method or a dry and wet spinning method (hereinafter, both spinning methods are referred to as wet spinning) is known. A porous membrane obtained by such wet spinning may provide a high filtration flow rate and a good fractionation layer, and is thus suitable for a large amount of water treatment.
In production of a porous membrane using such wet spinning, a porous structure is adjusted by changing membrane production conditions such as composition and temperature of a film-forming dope and a solidification bath. For example, in order to adjust the viscosity of an undiluted solution in an appropriate range in membrane production, to achieve stabilization of the membrane production state, and to cause phase separation for forming a three-dimensional network structure superior in water permeability, a hydrophilic polymer is added as an additive in addition to a polymer that forms a membrane base. In many cases; as the hydrophilic polymer, a high-molecular-weight polymer such as polyethylene glycol or polyvinylpyrrolidone is used. Further, the hydrophilic polymer is removed from the membrane by cleaning or the like after the film-forming dope is immersed in a solidifying fluid.
As a production method of such a porous membrane using the hydrophilic polymer, PTL 1 discloses a method of solidifying, in a flow casting manner, a film-forming dope that contains four components of polyvinylidene fluoride, a good solvent of the polymer, a poor solvent of the polymer and a water-soluble polymer.
Further, PTL 2 discloses a method of individually measuring the kinematic viscosity and storage elastic modulus with respect to each of a polymer that forms a membrane base and a hydrophilic polymer and combining these polymers for use so that the values of the kinematic viscosities and the storage elastic moduli are equal to each other between the polymer that forms the membrane base and the hydrophilic polymer, in preparation of a film-forming dope where the polymer that forms the membrane base and the hydrophilic polymer are dissolved in a solvent.
According to these methods disclosed in PTLs 1 and 2, a membrane of a three-dimensional network structure having high water permeability and superior filtration performance may be achieved.